1. Technical Field
The present invention relates to a water jet propulsion apparatus for a ship, and more particularly to a propulsion apparatus having improved suction performance and navigating performance.
2. Background Art
A water jet propulsion apparatus is known and disclosed in, for example, Japanese Patent Laid-Open No. 5-270486. In that prior apparatus water is sucked into a suction opening in a bottom of a ship, the sucked water is pressurized by a horizontal impeller of a pump disposed above a surface of the water and the water is jetted to a position behind a stern of the ship so that the ship is propelled. Another water jet propulsion apparatus has been disclosed in, for example, Japanese Patent Publication No. 7-117076, which incorporates a volute casing disposed horizontally and within which an impeller is rotated to spirally swirl water sucked from a position below the bottom of the ship and to jet a swirl water flow to a position behind the stem of the ship. Both of these apparatuses have several disadvantages.
The water jet propulsion apparatus disclosed in Japanese Patent Laid-Open No. 5-270486 is designed so that the impeller of the pump is disposed above the surface of the water. Therefore, when the ship starts navigating, the internal portion of a pump casing must be at a negative pressure to lift water from below the surface of the water to the position of the impeller. This design requirement makes it difficult to begin movement of the ship.
Since the impeller is disposed apart from the bottom of the ship, a long passage in a suction portion of the impeller is required and thus, a long actual lift of water to the impeller is required and great resistance is generated in the suction portion. As a result, cavitation takes place when the ship is navigated at high speed.
The propulsion apparatus is secured to the ship at the suction portion, which is supported at the bottom of the ship, and at the discharge portion, which is supported at the stern of the ship. This structural requirement makes it difficult to align a main shaft of the impeller with the axis of a drive shaft of a motor. A deviation between the alignment of the two shafts must be absorbed by force of some play in the system. Play in the system is provided by securing a projection portion to the stern. If the two shafts are connected to each other by an eccentricity, the main shaft, which is disposed horizontally, is deflected by the force of the weight of the impeller and vibrations of the motor are transmitted to the main shaft. This causes the rotating impeller to contact the bottom of the pump casing causing the impeller to be worn. Thus, there is apprehension that an adverse influence is exerted on the efficiency of the pump.
The water jet propulsion apparatus disclosed in Japanese Patent Publication No. 7-117076 is designed so that the volute pump casing is disposed horizontally. Therefore, if the ship is separated from the surface of water because of waves air can be sucked together with water into the apparatus, and the air cannot easily be discharged.
The trapped air generates eddy currents of air causing cavitation to take place. As a result of the cavitation, the propelling performance of the apparatus deteriorates.
The present invention is designed to solve the abovementioned problems, and an objective of the present invention is to provide a water jet propulsion apparatus that is capable of reducing the resistance which arises when water is introduced into the apparatus, reducing the cavitation that occurs when a ship is navigated at high speed, and that can easily be mounted to a ship.